You can only set boundaries for yourself. You can't change people's behavior.

I think you can say "In the future, I will be available to sit with Grandpa when you need me too. If cousin is there, it will be $10 an hour because sitting with him is work."

This way they will think about a) leaving cousin with you at all and b) how long they are gone. Right now they have no incentive to get back sooner.

OP, I agree with bopper. Set a price for watching the child and stick to it, regardless of his behavior that particular day. Make it enough that his mother will actually _think_ about the time as money spent. It may get her home sooner. Either that or decline watching the child and enjoy your time with your grandpa. It would still be courteous of grandma and aunt to stay within the schedule originally laid out, barring any complications, and to call you if any necessary hold-ups occur. Your time is valuable and should be respected in any case.

If they call again for help, be honest and tell grandma that you will be happy to stay with grandpa, but that you will not take care of the child as he did not listen to you last time and they must come home directly after the appointment. If she tries to say he will be good this time, etc, etc, or they need to run errands, stay firm and repeat "I will stay with grandpa but child cannot stay. Also, you will need to return home as soon as your appointment is over, no shopping, as I have other commitments. If that won't work for you, you will have to find someone else."

Repeat, repeat, repeat.

If you get there and they try to leave child with you, gather up your things and prepare to leave. Do so if you need to. Also remind them that they need to come right home. If they offer to have someone else come in after xxx hours, take them up on it. Get the number of the person and contact them yourself to make sure it is really going to happen.

It is great to help out your grandparents, but that doesn't mean that you need to be taken advantage of or watch a child who will not listen and creates stress for you.

They were so inconsiderate. They should have called and asked if it was okay and came home if you said no.

My brother is continually like this. He inconviences others and never says he is sorry. At the beginning of December we all get together so my parents can take a Christmas photo with the grandkids. He showed up 1.25 hours late and didn't even apologize. Another time I went to his house to pick up something he was making for me. When I got there, he was still in bed and it wasn't made. (Only took 20 minutes but still).